1. Field of the Invention
The present invention relates to a rotational displacement detection apparatus. The present invention is suitable for, e.g., a rotary encoder for detecting relative rotational displacement information between a main scale and a flat board or rotational displacement information of, e.g., an origin position by irradiating a light beam onto a radial grating on the main scale which rotates relatively, and an index scale (radial grating) attached to the flat board, and detecting phase- or intensity-modulated signal light coming from these scales.
In particular, the present invention is suitable for a so-called built-in type rotary encoder in which a disk unit for fixing the main scale is independent from a main body unit for fixing a light source means, a light-receiving means, and the index scale.
2. Related Background Art
Conventionally, as an apparatus for measuring relative rotational displacement information (a displacement amount, velocity, acceleration, or the like) of an object with high precision, a rotary encoder (to be simply referred to as an "encoder" hereinafter) is popularly used. Also, this encoder is added with a means for detecting origin information in order to calculate the absolute position information of rotation information.
FIG. 1 is a schematic sectional view of a conventional encoder.
Referring to FIG. 1, in a detection mechanism of an incremental signal (A and B phases) in the encoder, repetitive radial grating patterns 4 and 4Z of transmission and non-transmission (or reflection and non-reflection) portions are recorded on a main scale 3 fixed to a disk hub 8, which rotates relatively, and radial grating patterns 5A and 5B which have the same pitches as those of the patterns 4 and 4Z and have a spatial phase difference of 90.degree. therebetween are recorded on a stationary flat board (index scale) 5. After the main scale 3 and the index scale 5 are stacked to have a predetermined gap therebetween, a light beam coming from an LED 1 is irradiated onto these scales via a collimator lens 2 as a collimated light beam.
At this time, the amount of transmitted light periodically changes in correspondence with the degree of coincidence between the patterns on the two scales upon movement of the main scale. Changes in light amount at that time are detected by a light-receiving element 6 (6A and 6B) arranged on a base member 9, thus obtaining an electrical incremental signal having a sine waveform. Furthermore, the incremental signal is converted into a rectangular waveform by a binarization circuit, thus obtaining an electrical incremental signal. In this manner, rotation information of a rotation shaft 17 is detected.
On the other hand, in a detection mechanism of an origin signal (Z phase), the radial grating pattern 4Z consisting of a plurality of transmission and non-transmission (or reflection and non-reflection) portions is recorded on the main scale 3 which moves relatively, and a radial grating pattern 5Z which is identical to the pattern 4Z is also recorded on the stationary index pattern 5. After the two scales are stacked to have a predetermined gap therebetween, a collimated light is irradiated onto the two scales. Pulse-shaped signal light, which has a maximum transmitted light amount at an instance when the patterns on the two scales perfectly coincide with each other upon movement of the main scale 3, is obtained. The pulse-shaped signal light is detected by a light-receiving element 6 (6Z) arranged on the base member 9, thereby obtaining an origin signal. Furthermore, the origin signal is converted into a rectangular waveform to obtain an electrical origin signal.
Both the radial grating pattern and origin pattern are formed on each of the main scale 3 and index scale 5 used for detecting relative rotational displacement information. In many cases, both the incremental signal and origin signal are concurrently and parallelly detected using a single optical system. In this case, the detection principles of both the incremental and origin signals use the modulation effect of the transmitted light amount caused by changes in degree of overlapping between the main scale 3 and the index scale 5.
Recent encoders are required to attain a size reduction of the entire apparatus and detection of high-resolution rotational displacement information. In particular, the size reduction requirement requires not only a reduction of the size of the encoder main body but also a reduction of the length in the axial direction after the encoder is attached to a rotary member such as a motor. In order to meet such a requirement, a so-called "built-in type" encoder which does not have any rotation shaft is required. In this encoder, a disk is directly attached to the rotation shaft of, e.g., a motor, and thereafter, the encoder main body is assembled to the motor housing. Also, in this built-in type encoder, the disk (main scale) and the encoder main body (detection head) portion are spatially separated from each other.
When the user (measurement person) attaches such an encoder to, e.g., a motor, a process of fixing the disk to the rotation shaft of the motor and a process of fixing the encoder main body to the motor housing are required.
In this case, in order to accurately output A-, B-, and Z-phase signals from the encoder, the following conditions must be satisfied:
(a-1) the radial grating tracks on the disk do not decenter during rotation of the motor shaft; and PA1 (b-2) the radial grating tracks on the disk must perfectly overlap those on a parallel board (index scale) in the encoder main body.
In particular, when the encoder main body is attached to the motor housing, it is difficult, in practice, to form a high-precision fitting butt portion on the motor housing. When the position of the encoder main body is determined to have an intermediate-precision butt relationship and is fixed to the motor housing, a gap of about 100 .mu.m is formed. As a result, the attachment position of the encoder main body may be displaced by about 50 .mu.m in the x- or y-axis direction.
A case will be examined below wherein a disk 3 is decentered by 35 .mu.m in the x-axis direction, and the encoder main body with an index scale 5 is displaced by -50 .mu.m (a flat board (index scale) 5 is displaced by -50 .mu.m) in the x-axis direction.
A radial grating 4 on the disk 3 has a radius of 10 mm and a grating pitch of 25 .mu.m. An amplitude grating 4Z for origin detection, which is defined by random pitches on the disk 3, is assumed to be recorded within the radial position range of 6 to 8 mm. The disk 3 and the flat board 5 are displaced by 85 .mu.m in the x-axis direction as the sum of the decentering amount of the disk 3 and the attachment position errors of the encoder main body.
If an A-phase signal is detected at the radial position of 10 mm on the disk, the A-phase detection timing shifts by 85/25=3.4 cycles. If a Z-phase signal is detected at the radial position of 7 mm on the disk, the Z-phase detection timing shifts by 85/25.times.7/10=2.38. The relative phase shift between the A and Z phases is 3.4-2.38=1.02 cycles (note that the A-phase rectangular wave signal is used as a reference for one cycle). More specifically, the Z-phase waveform shifts by .+-.1.02 cycles or equivalent with respect to the A-phase waveform by the relative displacement of 85 .mu.m in the x-axis direction of the encoder main body and the disk.
In this case, it is difficult to synchronize (lock) the Z and A phases using a logic circuit.
As the disk 3 of the encoder is made compact (to have a smaller diameter) and the resolution becomes higher, the pitches of the radial gratings on the disk 3 and the flat board 5 become very small, and the relative positional displacement (the azimuth shift between the gratings) readily decreases the signal output. As a result, it is very difficult to set the built-in type encoder, and it is also very difficult to put such an encoder into practical applications.